When Zenyatta runs in the Apple Blossom Invitational on Friday, she'll be going for her 16th straight victory, which would tie a record held by Citation and Cigar. Last November, she became the first mare to win the Breeders' Cup Classic, defeating a field full of males, including Kentucky Derby winner Mine That Bird. How is it that females can beat males at the racetrack—and why don't they do it more often?
There is less sexual dimorphism among horses than humans. The average colt is around 10 percent heavier than a filly, and they're very close in height. Studies of running, rowing, speed skating, and swimming races have shown that human males are on average 11 percent faster than women. The gap between colts and fillies—male and female horses younger than 4 years old—is around 1 percent. According to handicapper Andrew Beyer, inventor of the Beyer Speed Figure, the average winner of the fillies-only Kentucky Oaks is five lengths—or one second—slower than the average Kentucky Derby winner.
It's possible that there's an evolutionary reason for this rough parity in speed. In the wild, both male and female horses must be able to run quickly and with endurance in order to escape from predators. In a paper titled "Gender Difference in Running Speed: Humans Versus Horses and Dogs," professor Pauline Entin of Northern Arizona University argues that "[g]iven the evolution of the horse as a prey species dependent on running … it is tempting to speculate that natural selection operated on the running ability of both males and females of these species."
Humans, on the other hand, "may have had gender-specific tasks at least as much as a million years ago, possibly lessening the importance of running speed, particularly in females," Entin argues. In an additional paper (PDF), Entin and a team of Belgian researchers write that "many scholars have linked the division of labor between the sexes in modern man to performance differences, arguing that hunting requires men's strength and speed more than other food-collecting tasks."
Considering the small speed difference between male and female equines, an extraordinary female can beat all the male competition. Despite the pre-Zenyatta shutout at the Breeders' Cup Classic, it's not unusual for females to win big races. Rachel Alexandra won last year's Preakness. Goldikova captured the 2008 Breeders' Cup Mile. At the Prix de l'Arc de Triomphe, Europe's most prestigious horse race, a filly or mare has triumphed in 16 of 88 runnings.
Parity between the sexes isn't limited to horses. Among racing pigeons, males are only 2 percent faster than females. Dogs are even more progressive. Male and female greyhounds always race against one another. In 2006, a bitch named Greys Calibrator won the first million-dollar greyhound race, the Derby Lane Million.
Why don't fillies challenge males more often? Because there's not a lot of financial incentive. Race cards are divided into contests strictly for fillies and mares, and "open company," for horses of either sex. A female horse is more likely win—and to take home whatever prize money is at stake—in a single-sex race.
In Thoroughbred racing, the real money is in breeding. Winning the Preakness didn't do much to increase Rachel Alexandra's value as a broodmare. She'd already proved she was the best female of her generation, and a mare can give birth to only one foal a year while a stallion can sire 150.
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Explainer thanks Roy L. Caldwell of the University of California, Berkeley.